In recent years, non-aqueous electrolyte secondary batteries that are charged and discharged as Li ions move between the negative electrode and the positive electrode have become a subject of active research and development as high energy density batteries. Such non-aqueous electrolyte secondary batteries are anticipated as large-sized power sources particularly for electric cars or hybrid cars that combine an engine and a motor, from the viewpoint of environmental problems. In addition, their use is not limited to the automobile applications, much attention is paid to the non-aqueous electrolyte secondary batteries also as electric power sources.
In such non-aqueous electrolyte secondary batteries, the characteristics of instantaneously releasing a large current are considered important to an extent equal to or higher than that of those non-aqueous electrolyte batteries used in small-sized mobile telephones, notebook PCs and the like.
Furthermore, as the positive electrode, attention is being paid to lithium iron phosphate (LiFePO4) having an olivine type structure, from the viewpoint of resources, and from the viewpoints of high environmental compatibility as well as electrochemical stability, thermal safety and the like.
A battery which uses lithium titanium composite oxide in the negative electrode, and lithium iron phosphate in the positive electrode, can become an innovative secondary battery that is far more safe and has a far longer service life as compared with conventional lithium ion secondary batteries.
However, lithium iron phosphate has a problem that the compound easily reacts with moisture, and iron is likely to be eluted from the positive electrode active material. Furthermore, the inventors confirmed that the lithium titanium composite oxide having a very fine particle size, which is used in the negative electrode, contains a large amount of water of crystallization. That is, in the case of such a combination of a positive electrode and a negative electrode, the large amount of water of crystallization contained in the negative electrode causes dissolution of lithium iron phosphate, deterioration of capacity is prone to occur, and the original potentials of the positive electrode and the negative electrode could not be sufficiently extracted.
A technique of coating, for example, a substance such as Li3PO4 that is inert to water, or carbon that has electron conductivity in order to suppress such reaction with moisture or the like, is already well known. However, since the substances mentioned as examples are not capable per se of insertion and extraction of lithium, the substances do not contribute to the charge-discharge capacity. Furthermore, at the equilibrium electrode potential at which lithium iron phosphate contributes to the reaction, the diffusion of lithium in Li3PO4 or carbon almost does not occur inside the solid. Therefore, since such coating serves as an inhibitory factor against the diffusion into lithium iron phosphate, the coating is not suitable for the applications where a large current is required.